expression: uncomment pushdown for JSONUnquote expression (#24504)

[ti-srebot]

cherry-pick #24504 to release-5.1
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/26265

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git push git@github.com:ti-srebot/tidb.git pr/26265:release-5.1-bd8d3b1a8cfb

---

What problem does this PR solve?

Issue Number: close #24415

Problem Summary:

What is changed and how it works?

This PR tries to open the pushdown of JSONUnquote expression, by uncommenting it in the 'pushdown list'.

Related changes

• Related doc to change: remove json_unquote() from expressions-pushed-down.md docs#5564
• Need to cherry-pick to the release branch

Check List

Tests

• Integration test(the "idc-jenkins-ci-tidb/integration-copr-test"): https://github.com/tikv/copr-test/blob/master/push-down-test/sql/randgen/9_json.sql

Release note

• Enable the pushdown of builtin function json_unquote() to TiKV.

[ti-srebot]

/run-all-tests

[ti-srebot]

@bb7133 you're already a collaborator in bot's repo.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• guo-shaoge
• wshwsh12

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[zhouqiang-cl]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 9991695

[zhouqiang-cl]

/merge

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

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/run-all-tests

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